There are a lot of devices that enhance lift generated by a wing at reduced speed, as slats, slots, flaps, but generally they do not provide any lift at zero aircraft speed. There are also well known vertical or short take off and landing (V/STOL) craft that adopts several methods for generating lift during VTOL operation, but each of them has certain disadvantages.
The most known hovering craft is the helicopter; to create lift it employs a rotor, that in order to achieve high efficiency in hover mode, it has a low disc loading, invariably leading to a large rotor, creating difficulties as the helicopter speed increases, such as retreating blade stall, high drag and loss of efficiency, making the helicopters unsuitable to operate at higher speed. A method to combat these deficiencies are employed by tilted rotor and tilted wing aircraft, such as Bell Boeing V-22 Osprey and Canadair CL-84. Their design is a compromise between hovering configuration efficiency, having higher disk loading than helicopters, and horizontal configuration efficiency, having more propeller disk that they need for generating forward thrust, resulting in more drag, compared to fixed wing aircraft. Another approach to eliminate retreating blade stall and to increase speed of a helicopter is employed by the compound helicopter, such as Piasecki X-49 and Eurocopter X3. This approach involves unloading the rotor disk at high speed, lift being provided partially by small wings, and having forward thrust provided by an auxiliary propulsion system. Although this method increases maximum speed of the compound helicopter, efficiencies, both in hovering and in forward flight, are reduced, because in each mode, there is an extra system, contributing little to the operation, leading to increased weight and drag.
Static lift generated by a propeller or fan is increased, if the propeller is enclosed into a shroud or a duct, tip losses are reduced, the shroud intake provides itself thrust, but although a shrouded propeller creates more static thrust, the drag created by the shroud becomes prohibitive as speed increases, and above a certain breakeven speed, the efficiency drops below of that provided by an open air propeller. A shroud optimized for high static thrust have a large bell shaped inlet, creating increased amount of drag, inherently inefficient at increased speed. An VTOL craft employing shrouded propellers to achieve VTOL flight is the experimental Bell X-22, but unable to achieve it's goal, the required maximum speed. Aircraft having shrouds optimized for high static trust are the Hiller VZ-1 Pawnee and the SoloTrek XFV. They were designed to operate exclusively in hover mode, inherently having a reduced transport efficiency.
Channel (Custer) wing type aircraft, as the CCW-5, have wings able to create lift at reduced speed, some test have shown they create an amount of lift even at zero speed. NACA tests of a channel winged aircraft shows less than 10% total thrust increase and lack of control at slow speed. It also suffers from vibration problems because the propeller blades have different loading in the proximity of the channel versus the open air.
In marine application, there are also devices augmenting propulsion system, but each of them are having certain disadvantages. Devices for increasing propeller thrust, as Kurt nozzles or accelerating ducts, are functioning optimum in certain conditions and designed speed. Major disadvantages are increased drag and cavitation as boat speed increases, and decreased efficiency. Debris and ice can be jammed between the propeller and the nozzle, and are much more difficult to clear than open propellers. Another type of devices used for augmenting propulsion are the decelerating ducts, used for reducing cavitation and noise, for high speed applications. They have certain disadvantages as well, the biggest disadvantage is efficient operation around a limited speed range, reduced thrust, increased drag and decreased efficiency. Debris and ice can be jammed between the propeller and the nozzle as well, the same as for Kurt nozzles.
There is a definite need for improvement, a need for a system that augments thrust and provides control at reduced speed, yet ensuring low drag at increased speed.